Attenuator for centimeter waves



sept. 9, 1947,

v. D. LANDON ATTENUATOR FOR CENTIMETER WAVES Filed oct. ze, 1945 nventor NADLANDUN (Ittorneg VERNU Patented Sept. 9, 1947 ATTENUATOR FOR CEN TIMETER WAVES Vernon D; Landon, Princeton,

N. J., assigner to Radio Corporation of America, a corporation of Delaware Application October 28, 1943, Serial No. 508,034

8 Claims. 1

This invention relates generally to centimeter wave apparatus and more particularly to an improved method of and means for attenuating microwave energy in a waveguide transmission system.

Various types of attenuators have been employed heretofore for controlling microwave energy in waveguide transmission systems. One of the commonest Vforms of microwave attenuator is an adjustable iris diaphragm disposed transversely of a waveguide effectively to provide a short section of waveguide operating below cuton". One of the principal disadvantages of any type of attenuator which employs movable metallic elements is that erratic contact between the conductive elements disturbs the normal current distribution in the waveguide system. The instant invention contemplates the use of a movable dielectric block proportioned so that it has substantially no effect upon the tuning of the waveguide system but varies the standing wave pattern therein with respect to a xed iris coupling aperture.

Briefly, a preferred embodiment of the invention contemplates means for establishing standing waves in a full wave mode within a resonant cavity, one of the physical dimensions of which is of the order of .707 wavelength at the operating microwave frequency and the longitudinal dimension of which is o-f the order of 1.06 Wavelength. A iixed iris aperture in one wall of the resonant cavity, centered .353 wavelength from one of the end walls of the cavity, opens into a waveguide which transmits microwave energy to a load device. A block of polystyrene, or similar low-loss dielectric material, is proportioned, in view of its dielectric constant, to have the same cross-sectional dimensions as the cavity and to have ka physical length of the order of .353 wavelength, corresponding to an electrica] length of 1/2 wavelength. The physical dimensions given are for an operating frequency of 10,000 megacycles, selected only for the purpose of illustration. These dimensions vary according to frequency in a manner known in the art.

The tuning of the cavity will not appreciably be affected as the polystyrene block is moved longitudinally along the cavity and across the iris aperture, since the block has an effective electrical length of 1/2 wavelength. However, interposing the higher dielectric constant material between the iris aperture and the adjacent end cavity wall changes the electrical distance therebetween from a quarter Wavelength to a half wavelength at the operating frequency. The result- VThe electrical wavelength in the cavity with an air or other iluid dielectric will `have an integral ratio, such as 2, as compared with the wavelength in the solid dielectric such as polystyrene. In an open or coaxial line the wavelength ratio would vary according to the square roots of the dielectric constants. However, in a cavity resonator or waveguide, the ratio is more closely the integral ratio of the dielectric constants since the walls of the cavity increase the air wavelength more than they increase the solid dielectric wavelength.

It should be understood that the term fluid dielectric employed herein is intended to include all liquids or gases having relatively high electrical insulating characteristics and dielectric qualities.

Among the objects of the invention are to provide an improved method of and means for attenuating centimeter waves. Another object of the invention is to provide an improved micro- Wave attenuator wherein the standing wave pattern in a resonant chamber is varied by adjusting the position of a dielectric element with respect to a xed iris aperture in the Wall of the chamber. A further object of the invention is to provide an improved microwave attenuator, or coupling adjustment, having low tuning effect upon a resonant wave transmission system. An additional object of the invention is to provide an improved method of and means for employing a movable dielectric element in a waveguide transmission system eifectively to change the standing wave pattern within said system for adjusting the energy transmission through a fixed iris aperture. g

The invention will be further described by reference to the accompanying drawing oi' which the single ligure is a perspective view of a preferred embodiment thereof.

Referring to the drawing, a microwave generator I is coupled through a transmissionline 3.

to a coupling loop 5 disposed Within a cavity resonator 1. The cavity resonator l may have a physical length, for example, of 1.06 wavelength, a width of .707 wavelength and a height of the order of 1A; wavelength at the operating microwave frequency, in terms of the wavelength in free space. The mode of oscillation in the guide is then such that the eiective electrical distance along the length, width, and height of the cavity are respectively 1, 1/2 and 0 wavelengths. A fixed aperture in one of the longitudinal narrow faces I I of the cavity resonator 1 opens into the end of a conventional rectangular waveguide I3 which may be connected to any desired type of load device, not shown. Since the electrical distance between the aperture 9 and the adjacent end cavity wall 2| is of the order of 1A; wavelength at the operating frequency, the standing wave pattern within the cavity will provide maximum coupling to the waveguide I3 through the aperture 9 when a fluid dielectric such, for example, as air, occupies the space between the aperture 9 and the adjacent end cavity resonator wall 2l, since the end wall may be considered to be at Aground or zero potential.

A solid dielectric block I1 is disposed Within the cavity resonator l and proportioned to fill entirely the space between the opposite longitudinal walls of the cavity resonator for a physical length of .353 wavelength (or an equivalent electrical length of 1/2 wavelength), along the longitudinal axis of the cavity resonator, The dielectric block preferably should be of polystyrene, or similai` low-loss dielectric material, having a dielectric constant of the order of 2.6. It will be seen that the portion of the cavity resonator 'I occupied by the dielectric block I'I will have an eiective electrical length of 1/2 wavelength because of the relatively higher dielectric constant of the block than that of the fluid dielectric.

It should be understood that the electrical quarter-wave cavity resonator dimension and the dielectric constant of the movable dielectric block, as well as the dielectric constant of the fluid dielectric within the cavity resonator, all may be varied in accordance with known microwave practice in any manner which will permit the dielectric element t change the effective distance between the center of the aperture and the adjacent end wall of the cavity resonator from some predetermined odd multiple of 1/4 wavelength yto some predetermined multiple of 1/2 wavelength at the operating microwave frequency as the dielectric element position is varied. The dielectric element should preferably be proportioned so that its effective electrical length is some multiple of 1/2 wavelength, in order that its effect upon the cavity resonator tuning may be minimized as the element is movedf'longitudinally along the cavity.

Athreaded control rod I9 is securely fastened to one of the vertical faces of the dielectric block I'I and is extended out through the adjacent end cavity resonator wall 2| and through a mounting bracket 23. The end wall 2| may be relativeli heavy to provide a bearing surface for the rod. An adjusting thumb nut 25, disposed within a slot 2l in the mounting bracket 23, is threaded to engage the threads of the control rod I9, whereby rotation of the thumb nut 25 provides longitudinal displacement of the dielectric block I1 along the interior of the cavity resonator l. It should be understood that any other known means may be employed for adjusting the position of the dielectric block within the cavity resonator.V

Minimum attenuation or maximum energy transfer through the iris aperture 9, will be provided when no portion of the dielectric block I 'I is interposed between the iris aperture 9 and the adjacent end cavity resonator wall 2|. As the dielectric block I'I is progressively interposed between the iris aperture 9 and the adjacent end cavity wall 2|, the space between the iris aperture and the end wall becomes increasingly closer to 1/2 the electrical wavelength, and the effective attenuation increases accordingly. Minimum energy transmission through the aperture, or maximum attenuation, is provided when the dielectric blocl: entirely occupies the space between the aperture and the adjacent end cavity wall 2|. The maximum and minimum values of attenuation which may be provided with the device thus disclosed may be varied by suitably proportioning the dimensions of the iris aperture 9 in accordance with accepted microwave practice.

Thus the invention disclosed comprises an improved method of and means for attenuating microwave energy in a resonant microwave transmission system wherein the position of a movable dielectric block is varied within a resonant cavity to vary the microwave standing wave pattern therein and hence the energy transmission through a fixed iris aperture in one of the cavity walls.

I claim as my invention:

l. Apparatus for adjustably coupling microwave energy to a waveguide transmission system including a cavity resonator having a fluid dielectric and having at least one microwave permeable aperture therein, a waveguide coupled to said resonator through said aperture, means for establishing standing microwaves in said resonator, a relatively higher constant dielectric element entirely disposed in said resonator and means for changing the position of said element between said aperture and one of the walls of said resonator to vary the standing wave pattern therein and hence the effective microwave coupling through said aperture between saidresonator and said waveguide.

2. Apparatus for adjustably coupling microwave energy to a waveguide transmission system including a cavity resonator having a wall including a microwave permeable aperture, said aperture being separated by a predetermined odd multiple of one quarter wavelengths in a iiuid dielectric at the operating microwave frequency from one of the adjacent resonator walls, a waveguide coupled to said resonator through said aperture, means for establishing standing microwaves in said resonator, a relatively higher constant dielectric element disposed in said resonator, and means for adjustably interposing said element between said aperture and said adjacent resonator wall for varying the standing wave pattern within said resonator and hence the effective microwave coupling through said aperture between said resonator and said waveguide.

3. Apparatus for adjustably coupling microwave energy to a waveguide transmission system including a cavity resonator having a uid dielectric and having at least one microwave permeable aperture therein, a waveguide coupled to said resonator through said aperture, means for establishing standing microwaves in said resonator, a relatively higher constant dielectric for changing the position of said element be-y tween said aperture and one of the walls of said resonator to vary the standing wave pattern therein and hence the effective microwave coupling through said aperture between said resonator and said waveguide.

4. Apparatus for adjustably coupling microwave energy to a waveguide transmission system including a cavity resonator having a wall including a microwave permeable aperture, said aperture being separated by a predetermined odd multiple of one quarter wavelengths in a fluid dielectric at the operating microwave frequency from one of the adjacent resonator walls, a waveguide coupled to said resonator through said aperture, means for establishing standing microwaves in said resonator, a relatively higher constant dielectric element having an effective length of one half wavelength disposed in said resonator, and means for adjustably interposing said element between said aperture and said adjacent resonator wall for varying the standing wave pattern within said resonator and hence the effective microwave coupling through said aperture between said resonator and said waveguide.

5. Apparatus for adjustably coupling microwave energy to a waveguide transmission system including a cavity resonator having a wall including a microwave permeable aperture, said aperture being separated by a predetermined odd multiple of one quarter wavelengths in a fluid dielectric at the operating microwave frequency from one of the adjacent walls of said resonator, a waveguide coupled to said resonator through said aperture, means for establishing standing microwaves in said resonator, an element having a dielectric constant substantially some even multiple of the dielectric constant of said uid and having an effective length of some even multiple of one half wavelength disposed in said resonator, and means for adjustably interposing said element between said aperture and said adjacent resonator wall for varying the standing wave pattern within said resonator and hence the effective microwave coupling through said aperture between said resonator and said waveguide.

6. Apparatus for adjustably coupling microwave energy to a waveguide transmission system including a cavity resonator having a wall including a microwave permeable aperture, said aperture being separated by a predetermined odd multiple of one quarter wavelengths in a uid dielectric at the operating microwave frequency from one of the adjacent walls of said resonator, a waveguide coupled to said resonator through said aperture, means for establishing standing microwaves in said resonator, an element having a dielectric constant substantially some even multiple of the dielectric constant of said fluid and having an eiiective length ci some even multiple of one half wavelength disposed in said resonator, and externally operable means for adjustably interposing said element between said aperture and said adjacent resonator wall for varying the standing wave pattern within said resonator and hence the effective microwave coupling between said resonator and said waveguide.

'7. Apparatus of the type described in claim 1 including means operable from without said resonator for actuating said element position chang- .ing means.

8. Apparatus of the type described in claim 6 including means operable from without said resonator for actuating said element adjusting means.

VERNON D. LANDON.

Name Date Hansen et al Oct. 21, 1941 Dallenbach May 7, 1940 Ginzton Sept. 10, 1946 Number 

